A collection of interesting articles I've gathered from different sources...
Koko (austerity):
kokoKoko emphasizes restraint, exclusion, and omission. The goal is to present something that both appears spare and imparts a sense of focus and clarity. In creating beautiful works of Bonsai, it is inevitable that the artist’s personality and character will come across in its appearance. Much like canvas art, every element will provide the audience with an insight into the artist’s soul. Strive to eliminate what is not needed in the design. It has been my experience that this element is the first and many times the most difficult to achieve for many artists.

Refrain from leaving/adding what is not absolutely necessary in the first place